1. Field of the Invention
The present invention relates to a drive circuit for a load, which can be used in image forming apparatuses, represented by a television receiver, a digital camera, a digital video camera recorder, a monitor of a computer, a printer of an electrophotographic system, and the like. More specifically, the invention relates to a drive circuit for a light-emitting element that can be used in a display and an exposing device that uses a light-emitting element as a load.
2. Related Background Art
As an example of a load, a light-emitting element, in particular, an organic EL (electroluminescent) element is a planar self-luminous element of stacked thin film layers, which is capable of emitting light at a high luminance. This EL element makes it possible to emit light at a low voltage and high efficiency by increasing the number of functional stacked layers of organic layers (see “Applied Physics Letters” Vol 51, 1987, page 913, and “Journal of Applied Physics” Vol. 65, 1989, page 3610). Since the organic EL element can obtain a substantially linear light-emitting intensity with respect to an electrical current, a constant current drive method has been proposed.
FIG. 8 shows an example of a circuit structure of one pixel of a display element using the conventional EL element. In the figure, reference numerals 1, 3 and 4 denote thin film transistors (TFT); reference numeral 2, a capacitor; reference numeral 5, an EL element, reference numeral 6, an ammeter; and reference numeral 7, a power supply. An operation of the circuit will be explained with reference to a timing chart of FIG. 9.
In a predetermined writing period, a source potential Vsig of the n-type TFT 1 is set to a display signal corresponding to a luminance of a display of the pixel in the next frame, a gate potential Vg1 of the TFT 1 rises to H (high level) at time t1 at which the signal is decided, as shown in FIG. 9, and the TFT 1 is turned ON, whereby a charge corresponding to the display signal is accumulated in the capacitor 2. Subsequently, Vg1 falls to L (low level) at t2 and the TFT 1 turns OFF again and, at the same time, a gate voltage Vg2 of the n-type TFT 4 rises to H and the TFT 4 is turned ON. Thus, an electrical current (display current) corresponding to the charge accumulated in the capacitor 2 flows to the TFT 3 to be supplied to the EL element 5, whereby the EL element 5 emits light at a luminance corresponding to the display signal until the next writing is performed. Reference numeral 6 denotes an ammeter, which is unnecessary for an actual drive circuit, but is illustrated here for the explanation of operation.
However, it is known that, even if the organic EL element emits light at a constant current, an impedance changes due to deterioration of stacked organic layers and a luminance falls with time, as shown in FIG. 10. FIG. 10 shows a rough tendency, and actual aged deterioration of characteristics of the organic EL element is not limited to that shown in the figure.
Thus, a method of measuring a drive time to change a luminance and a method of detecting a luminance with a sensor to adjust a drive voltage have been proposed (see Japanese Patent Application Laid-Open No. S59-055487).
Japanese Patent Application Laid-Open No. S59-055487, “Applied Physics Letters” Vol. 51, 1987, page 913, and “Journal of Applied Physics” Vol. 65, 1989, page 3610, propose methods for coping with the fall in luminance due to deterioration of the organic EL element. However, according to these proposals, the methods require means for storing a drive time and a sensor, and it is difficult to compensate for a change in a luminance by a unit of a frame for each pixel.